The embodiments disclosed relate generally to differential pressure valves, i.e. to valves which are automatically actuated by the pressure difference across the valve. Some exemplary embodiments specifically relate to reciprocating compressor valves, such as in particular to poppet valves or hyper compressors.
Hyper compressors, those capable of producing gas pressure levels up to or above 3,000 bars, are widely used in industrial application, including, but not limited to, the production of low density polyethylene, or LDPE. The efficient performance of these compressors is controlled at least in part by suction and discharge automatic poppet valves. FIG. 1 illustrates a cutaway of a portion of a hyper compressor 2 of the conventional art comprising two poppet valves 10. FIG. 2 illustrates an enlarged section of one of the conventional poppet valves of the compressor shown in FIG. 1, in an opened position. A poppet guide according to the state of the art is disclosed in US-A-2010/0024891.
As shown in FIG. 1, a hyper compressor 2 usually comprises a casing 3 in which a cylinder 4 is formed. A piston rod 5 slides reciprocatingly in the cylinder 4 to suck a fluid from a suction duct 6 and discharge the fluid at a higher pressure in a discharge duct 7. A poppet valve 10 is arranged in each said suction duct 6 and discharge duct 7. In FIG. 1 reference number 10S designates the poppet valve in the suction duct 6 and reference number 10D designates the poppet valve in the discharge duct 7. Each poppet valve 10S, 10D is designed as shown in FIG. 2 and is designated 10 therein.
As shown in FIG. 2, the conventional poppet valve 10 includes a valve body 11 that contains therein a poppet, or poppet shutter, 12 configured to open and close the gas flow path in and out of the hyper compressor 1, a spring 14 configured to keep the poppet shutter 12 in a closed position against a closure seat 13 formed by a portion of the internal surface of the valve body 11, and a shutter guide 16 that contains the poppet shutter 12 and the spring 14. As shown, when the poppet shutter 12 is forced opened, a flow passage 17 (identified by several arrows in FIG. 2) is formed from a flow inlet 18 to a flow outlet 20 of the conventional poppet valve 10, the flow path being defined by the space formed between the poppet shutter 12 and the valve body 11 as well as between the shutter guide 16 and the valve body 11. The shutter guide 16 of the conventional poppet valve 10 further includes a discharge opening 22 along an axis A-A of the shutter guide 16 connecting an inside chamber 26 of the shutter guide 16 to the flow passage 17 in a region of flow stagnation, the back pressure in the inside chamber 26 being defined at least in part by the static pressure in the region of the flow passage 17 around the axis A-A of conventional poppet valve 10.
Opening and closing of the poppet valves 10, 10S, 10D is automatically controlled by differential pressure across the valves. These valves are therefore sometime called “automatic valves” and distinguish over controlled valves, such as those commonly used in internal combustion engines, where valve opening and closing is controlled by an external actuator, such by way of a cam shaft.
The suction poppet valve 10S is arranged such that it opens when the pressure in the cylinder 4 of the hyper compressor 2 diminishes during the suction stroke of the piston rod 5. The pressure in the suction duct 6 overcomes the force of the spring 14; the differential pressure across the valve causes opening of the valve and fluid is sucked in the compressor cylinder 4. The discharge valve 10D is closed. Once the piston rod 5 reaches the bottom dead center, the movement is reversed and compression of the fluid in the cylinder starts. Increased pressure in the cylinder 4 automatically closes the suction valve 10S and opens the discharge valve 10D when the differential pressure across the discharge valve 10D, between the compressor cylinder 4 and the discharge duct 7, overcomes the force of the relevant spring.
At each closing movement the poppet shutter 12 of the relevant poppet valve 10, 10S, 10D strikes violently against the seat 13 of the valve body 11 and each opening stroke causes the poppet shutter 12 to strike against the shutter guide 16.
These poppet valves play an important role in the reliability of hyper compressors used in plants for the production of LDPE. The performance of such valves depends not only on selected material properties and a suitable design to withstand high operating gas pressures, but also on a proper dynamic behavior of the poppet shutter 12. The proper opening and closing of the valve are influenced by various design constraints related to several dynamic forces acting on the valve, including a drag force acting on the poppet shutter 12 and shutter guide 16 to open the valve, this drag force being generated by the interaction of the gas flow with the noted valve parts; a gas pressure force acting on the shutter guide 16 to close the conventional valve 10, this gas pressure force being generated by the flow back pressure acting on a back surface of the shutter guide 16; an inertia force associated with the mass of the poppet shutter 12; and a spring force generated by the spring 14 to close the valve, among others.
Hyper compressors operate usually in a speed range between 150 and 300 rpm. At each cycle all the valves perform an opening and closing movement with corresponding impacts of the poppet shutter against the seat 13 and against the shutter guide 16. Repeated impacts cause impact wear and frontal damages, which eventually lead to poppet failure. Impact wear causes material consumption and surface irregularities that can crate favorite sites for the formation of cracks. These can propagate by impact fatigue due to the stress waves generated by dynamic loads caused by impacts, till final fracture of the poppet shutter. In case of high impact velocities, impact fatigue can nucleate cracks itself, even in the absence of impact wear.
When there are impact loads on the springs, the stress propagates along the spring wire. The end coil of the spring in contact with the applied load takes up whole of the deflection and then it transmits a large part of its deflection to the adjacent coils. This wave of compression travels along the spring indefinitely. Resonance will occur depending upon time traveled. This results in very large deflections and correspondingly very high stresses. Under these conditions, it is just possible that the spring may fail. This phenomenon is called surge. From another view point, an impact stress applied on the spring upon closing or opening of the shutter introduces a deformation of the spring according to a function which can be decomposed in a Fourier series which also includes harmonics corresponding to the resonance frequencies of the spring. Under some circumstance this can generate the above mentioned compression wave traveling along the spring. The high stress induced in the spring by resonance can eventually result in spring failure. If this occurs in an automatic valve of a reciprocating compressor the shutter will continue to operate but under abnormal operating conditions. Impact velocities of the shutter increase to very high values giving rise to frontal damage (impact wear and impact fatigue) of the shutter. Frontal damage generate cracks which rapidly propagate under dynamic stresses produced by repeated impacts till final failure of the valve, when the shutter breaks.
Spring surge can be induced also by gas dynamic forces (vortex shedding). These forces generate pressure oscillations with a frequency typically ranging between 100 and 1200 Hz, corresponding to one or more resonance frequencies of the valve spring.
Automatic valves are used not only in hyper compressors but also in other kinds of reciprocating compressors, commonly used for lower pressure ranges, e.g. between 100 and 1000 bar. These automatic valves comprise a valve stop with one or more outlet apertures and a valve seat with one or more inlet apertures. Disk-shaped or ring-shaped shutters, or valve plates, are arranged between the seat and the counter-seat and are resiliently urged by springs against the valve seat. Opening and closing is controlled by the differential pressure across the valve. These valves are commonly called “ring valves”, to indicate the shape of the shutters used therein. Spring surge can arise also in this kind of automatic valves of reciprocating compressors, due to the effect of the impact loads on the spring upon opening and closing of the ring-shaped shutters.
It would therefore be desirable to develop an improved reciprocating-compressor valve, in particular an automatic valve, such as specifically a poppet valve for hyper compressors, where spring surge phenomena are suppressed or at least mitigated.